The present invention relates to electric power plants and more particularly to startup control systems for once-through boilers having integral separators.
It has been common to employ an off-stream flash tank for startup of a once-through boiler. For example, the boiler may be designed to provide 246.05 KGSC (kilograms per square centimeter) steam at the turbine during normal operation and the flash tank may be designed to supply steam at 70.3 KGSC for startup and operation of the turbine up to about 10% load. During all startups, a V valve between the primary and secondary superheater is closed, and P and N valves are open. Water is heated in furnace section and primary superheater passes and flows into the flash tank where the steam is separated from the water. The steam then flows back to the final superheater where additional heat is added before flowing to the turbine. The steam generated in this manner requires coordinated action to open the V valve, close a P valve, and modulate the burner firing rate and adjust the turbine valves to avoid interaction which could cause abrupt changes in steam temperature or pressure or turbine/generator output. Operation with this type system has been satisfactory for base loaded units, but the complex startup and shutdown procedure has made it unsuitable for cycling service.
More recently, an integral separator starting system has been developed for Foster-Wheeler once-through boilers, and it decreases the complexity of starting and increases the availability and cycling capacity of the once-through boiler. In this arrangement, the low pressure off-line flash tank is eliminated and a set of separators are installed in the main flow path upstream from the primary superheater. In this manner, provision is made for: (1) water to flow through the furnace tubes at a minimum rate to protect them from overheating; (2) diversion of water from passage to the turbine; and (3) a startup supply of steam until the boiler reaches its normal operating range.
In the integral separator startup system, a plurality of sets of valves are disposed upstream from the separators to control the waterwall pressure or the back pressure on the boiler feed pump as the boiler is ramped into operation. A set of W valves and a parallel set of Y valves have been coordinatively operated in the Foster-Wheeler boiler to control waterwall pressure as the boiler moves up to 25% load. Above 25% load, the W and Y valves are open.
An additional set of PPR valves have more recently been added in parallel to the W and Y valves to provide finer waterwall pressure control over a lower pressure operating range. For example, the PPR valves may provide pressure control up to 246.05 KGSC, and the W and Y valves may provide pressure control from 249.57 KGSC up to about 337.44 KGSC at 25% load.
Since the number of PPR valves may vary from plant to plant, it is desirable that a control designed to operate valves like the PPR valves be flexible for manufacturing variability while reliably providing the basic sequencing functions needed for pressure control.